The present invention relates to a knock control system and method for internal combustion engines. The invention is particularly applicable to spark ignition engines having electronic ignition.
It is now well known in computer or microprocessor controlled electronic ignition systems to store characteristic data for use in controlling operation of the engine. This may include an ignition base map which optimises the value of an ignition angle and dwell period for various operating conditions of the engine. The map stores optimised values for possible combinations of engine speed and engine load which can be modified to compensate for variations in engine temperature for example. The characteristic data is usually derived empirically, supplied by the manufacturer of the engine, and is obtained by running the engine under experimental conditions on a test stand.
In use, and as the engine wears, components age or are changed, valves and their adjustment change, the fuel quality changes and differs from that under which the tests were made and, as a result of these changes, the stored characteristics may no longer fit the engine and are less than ideally suited for its control.
Air pressure, ambient humidity and the like can also change the operation of an engine and affect the suitability of the stored characteristics.
Therefore, the stored characteristics alone are not sufficient to control the operation of the engine. In particular, if the ignition angle is not optimised for existing operating conditions, combustion knocking can occur. Combustion knocking can damage an internal combustion engine and should be eliminated if possible. Therefore, electronic ignition systems generally include a knock control system. Knocking can be eliminated by retarding the ignition angle but this can have an adverse effect on exhaust emissions and possibly fuel consumption. The ignition should ideally be retarded by the minimum amount sufficient to ensure that knocking does not occur, i.e. the engine should operate close to the knock limit. In practice knocking is detected by a sensor which detects engine vibrations and the knock intensity is determined from the sensor signals. In fact knocking is not eliminated entirely but rather an acceptable intensity band is defined within which the engine should be operated.
Thus, in an electronic ignition system with knock control, when the engine is at a particular operating point the electronic control unit (ECU) signals the ignition to be retarded in response to detection of knocking of unacceptable intensity. If, after retardation, unacceptable knocking is still detected the ignition will be retarded further and so on until the knocking is within the acceptable intensity band. When the operation of the engine is moved to another operating point, a new value for the ignition angle will be taken from the ignition map and this new value will be modified by the knock control system if necessary. If the detected knock intensity is below the acceptable intensity band the ignition timing may be brought forward by the knock control system until the knock intensity is within that band. This is known as closed-loop knock control.
A disadvantage with ordinary closed-loop knock control is that when the engine returns to an operating point at which knocking was previously occurring the knock control system has to repeat the process of modifying the ignition angle. Therefore, more recent knock control systems have a "learning" function. With "learning" knock control, as well as the base ignition map, a second "learning" map is stored which contains data for modifying the data in the base ignition map. The data in the learning map is accumulated during running of the engine. Thus, for example, if knocking is detected at a particular operating point, so that the ignition angle from the base map is modified by the closed loop knock control system, a difference value for the ignition angle is stored in the learning map so that when the engine returns to the same operating point the modified value for the ignition angle is used automatically, as derived from the base map together with the learning map, and it is not normally necessary for the closed loop knock control system to further modify the ignition angle. Other factors may effect engine knocking such as fuel quality and atmospheric pressure as mentioned above. If there is a large change in, say, atmospheric pressure before the engine returns to a particular operating point, the closed loop knock control may operate to alter the ignition angle derived from the base map and the learning map and the learning map will be updated.
A knock control system having a learning function is disclosed in U.S. Pat. No. 4,700,677. Such a system is generally called an adaptive system because the stored functional relationships which control ignition timing are adapted for the current operating conditions.
In known knock control systems having a learning function as described above, the learning map is updated after a predetermined time which may be defined as a particular number of combustion cycles. This predetermined time must be sufficiently large to establish normal running conditions, i.e. free from transients. On the other hand it must not be too large since otherwise the overall response of the engine to changes in conditions will be too slow. The time, which will hereinafter be referred to as the "observation time", is a fixed parameter of the learning algorithm. As a result, the learning algorithm is not able to adapt itself to various operating conditions of the engine resulting from changes in the manner in which the car is being driven. For example, one driver may drive the car very differently from another but known systems cannot be adapted to account for this. Also, the same driver may drive very differently on a motorway from the way he drives through a town. If a driver is particularly erratic, the engine may not remain at operating conditions sufficiently long for the learning map to be updated. The efficiency of the learning process is dependant on the driver, which is clearly undesirable.